1. Field of the Invention
The present invention concerns a picture tube having a video output intended to transform the picture of an incident radiation into an electrical signal.
In the following description, reference will be made more parituclarly to picture tubes having a video output utilized in radiology, i.e. to X-ray converter or intensifier tube. It is, however, obvious for the man skilled in the art that the invention can also apply to picture tubes detecting or converting radiations from within the visible spectrum, within the invisible spectrum such as X-rays or even a neutron stream. In this case, it is necessary to change the nature of the input screen in order to adapt it to the incident radiations to be converted.
2. Description of the Prior Art
In order to fully comprehend the problem that is overcome in the present invention FIGS. 1(a) and 1(b) represent two picture systems with video outputs utilized in radiology, namely a radiology picture intensifier tube having a video output and a system constituted by a picture intensifier tube optically connected to a vidicon tube.
The picture intensifier tube having a video output represented in FIG. 1(a) designated as a whole by the reference numeral 1 comprises, from left to right of the figure, the picture intensifier tube itself along with the picture taking part that are contained in the same vacuum enclosure 2. The vacuum enclosure 2 comprises an entrance window 4 transparent to the X-ray beam that is detected after having through crossed the body 3 to be observed.
The picture intensifier tube accordingly includes within the enclosure an entry screen consisting of flash or flicker device 5 and a photocathode 6 for conversion of the incident X-rays in turn into photons and photoelectrons g1, g2 and g3 for focusing and accelerating the emitted electrons towards the conical anode A. The tube further includes a target screen 7 one surface f1 of which receives the electrons emitted by photocathode 6 and the other surface f2 of which is scanned by an electron beam provided by thermoemissive cathode K heated by filament 8 and focused and accelerated by grids g4, g5, g6 and g7.
Output video signal S is, in this case, collected on the face f.sub.1 of the target 7.
The system represented in FIG. 1(b) comprises a picture intensifier tube T, an optical coupling system L and a vidicon tube V. The picture intensifier tube T is identical to the picture intensifier tube part represented in FIG. 1(a). The sole difference between these two parts lies in the fact that the picture intensifier tube T of figure 1(b) comprises an electroluminescent screen 7' upon which is formed the visible picture of the observed body. Similarly, the vidicon tube V is similar to the picture taking part of the tube represented in FIG. 1(a) and will therefore not be again described in detail, the same elements bearing identical references in both figures.
The main drawback of these two picture taking systems when they are utilized especially in radiology is their bulkiness, particularly for large picture field tubes. Indeed, in picture intensifier tubes, electronoptics do not allow very wise angular openings without a deterioration in the quality of the picture. This situation leads to selecting length/picture field ratios higher than 1.3:1. Similarly, in vidicon tubes, for electronoptics reasons, the length/picture field ratio is higher than 4:1. Consequently, the greater the picture image, the greater the depth of the system, even when, in the case of the system represented in FIG. 1(b), the optical coupling system L allows to place the vidicon tube V perpendicular to the picture intensifier tube T. By way of example, a useful field of 40.times.40 cm.sup.2 leads to a depth, for a traditional picture intensifier tube, higher than 75 cm.
Consequently, if it is required to produce a picture taking system having a wide field and low bulk in depth, it is necessary to utilize concepts different to those already utilized in the prior art.